1. Field of the Invention:
The present invention relates to a thin-film transistor array used in a liquid-crystal display device, and more particularly, to an improvement for decreasing the defective ratio of the thin-film transistor.
2. Description of the Related Art:
In recent years, with a progress of office automation, many attentions have been paid to the research and development of plane display devices such as liquid crystal display devices. The liquid crystal display device is constituted by interposing liquid crystal between a common electrode board and a thin-film transistor array board. The thin-film transistor array board has a matrix of thin-film transistors each of which accompanies a pixel electrode. For displaying a clear image, especially clear characters, a large number of pixel electrodes are required in a small area together with the accompanied thin-film transistors.
The thin film transistors are formed by covering a gate electrode with amorphous silicon layer via an insulating film and disposing source and drain electrodes on the amorphous silicon layer on both sides of the gate electrode. A video signal and a scan signal are respectively applied to the drain electrode and the gate electrode to sequentially apply the video signal to the pixel electrodes. An image is displayed by changing the light transparency of the liquid crystal on the pixel electrodes.
Here, it is very difficult to form a large number of thin-film transistors without any defective ones. Even if the thin-film transistors are formed with using the most advanced processing technology, it is hard to decrease the number of the defective transistors less than five among 250.times.250 thin-film transistors. Such defective transistors deteriorates the image quality. Especially, the displayed characters are made hard to be read.
Measures in the prior art for decreasing the number of defective thin-film transistors were usages of two series-connected or parallel-connected thin-film transistors in place of a single thin-film transistor, as shown in FIGS. 1(a) and 1(b).
According to the first measure shown in FIG. 1(a), a pair of thin-film transistors, one having a gate 1-1, a drain 1-2 and a source 1-3 and the other having a gate 2-1, a drain 2-2 and a source 2-3 are connected in series between a video signal line 6 and a pixel electrode 7. Both the gates 1-1 and 2-1 are connected to the same scanning line 5. The second measure shown in FIG. 1(b) connects a pair of thin-film transistors, one having a gate 11-1, a drain 11-2 and a source 11-3 and the other having a gate 12-1, a drain 12-2 and a source 12-3, in parallel between a video signal line 16 and a pixel electrode 17. Both the gates 11-1 and 12-1 are connected to the same scanning line 15.
There are, however, two types of defects. First type keeps a continuous "off" state. The measure shown in FIG. 1(a) is not effective against such first type of defect. Second type keeps a continuous "on" state. The other measure shown in FIG. 1(b) is not effective against such second type of defect. Therefore, the measures in the prior art are not effective to those two types of defects and do not effectively decrease the number of defective pixels in displayed image.